Modelling of aromatics and soot formation from large fuel molecules

There is a need for prediction models of soot particles and polycyclic aromatic hydrocarbons (PAHs) formation in parametric conditions prevailing in automotive engines: large fuel molecules and high pressure. A detailed kinetic mechanism able to predict the formation of benzene and PAHs up to four rings from C₂ fuels, recently complemented by consumption reactions of decane, was extended in this work to heptane and iso-octane oxidation. Species concentrations measured in rich, premixed flat flames and in a jet stirred reactor (JSR) were used to check the ability of the mechanism to accurately predict the formation of C₂ and C₃ intermediates and benzene at pressures ranging from 0.1 to 2.0 MPa. Pathways analyses show that propargyl recombination is the only significant route to benzene in rich heptane and iso-octane flames. When included as the first step of a soot particle formation model, the gas-phase kinetic mechanism predicts very accurately the final soot volume fraction measured in a rich decane flame at 0.1 MPa and in rich ethylene flames at 1.0 and 2.0 MPa.     

Modelling biodiesel–diesel spray combustion using multicomponent vaporization coupled with detailed fuel chemistry and soot models

A multicomponent vaporization model is integrated with detailed fuel chemistry and soot models for simulating biodiesel–diesel spray combustion. Biodiesel, a fuel mixture comprised of fatty-acid methyl esters, is an attractive alternative to diesel fuel for use in compression-ignition engines. Accurately modelling of the spray, vaporization, and combustion of the fuel mixture is critical to predicting engine performance using biodiesel. In this study, a discrete-component vaporization model was developed to simulate the vaporization of biodiesel drops. The model can predict differences in the vaporization rates of different fuel components. The model was validated by use of experimental data of the measured biodiesel drop size history and spray penetration data obtained from a constant-volume chamber. Gas phase chemical reactions were simulated using a detailed reaction mechanism that also includes PAH reactions leading to the production of soot precursors. A phenomenological multi-step soot model was utilized to predict soot emissions from biodiesel–diesel combustion. The soot model considered various steps of soot formation and destruction, such as soot inception, surface growth, coagulation, and PAH condensation, as well as oxidation by oxygen and hydroxyl-containing molecules. The overall numerical model was validated with experimental data on flame structure and soot distributions obtained from a constant-volume chamber. The model was also applied to predict combustion, soot and NOx emissions from a diesel engine using different biodiesel–diesel blends. The engine simulation results were further analysed to determine the soot emissions characteristics by use of biodiesel–diesel fuels.     

Simultaneous imaging of Mie scattering,PAHs laser induced fluorescence and soot laser induced incandescence to a lab-scale turbulent jet pulverized coal flame

In this study, transient soot formation processes in a small-scale jet burner (CRIEPI burner) were investigated by simultaneous measurements of coal particles, polycyclic aromatic hydrocarbons (PAHs) and soot. Pairs of simultaneous measurements of “Mie scattering measurement for coal particles with laser induced fluorescence (LIF) for PAHs” and “LIF for PAHs with laser induced incandescence (LII) for soot” were performed to understand the transitive formation processes of soot formation in pulverized coal flame, whose signals were successfully separated. Findings in the present study are as follows. Coal particles, PAHs and soot were distributed in this order in radial direction from the central axis. Existing regions of coal particles, PAHs and soot were overlapped from the time averaged viewpoint while there were few overlapping areas of coal particles, PAHs and soot from the instantaneous viewpoint. This result indicates that a long time is required for the formation of soot from 2 to 3 rings PAHs through larger PAHs.     

Hydrogen enrichment enhances soot formation in swirl-stabilized non-premixed turbulent combustion of ethylene in a model gas turbine combustor

A switch from fossil fuels to hydrogen is currently not feasible mostly due to supply and infrastructure issues. One of the possible approaches, and this is now practiced to a limited extent in industrial gas turbines, is to blend relatively small amounts of hydrogen with fossil fuels curbing the carbon dioxide emissions. However, studies assessing the influence of modest amounts of hydrogen blending with hydrocarbon fuels on soot processes yielded contradictory results. Most of these experimental and numerical studies were performed on laminar diffusion flames and studies on turbulent flames are scarce. One of the confounding factors in assessing the influence of hydrogen is selection of a control experiment in which the fossil fuel is blended with the same amount of an inert diluent. Using helium in the control experiment is preferable because of its similar transport properties and heat capacity to those of hydrogen. Hence, we studied the soot processes in a model gas turbine combustor in which the flame is stabilized by an air swirl. Swirl-stabilized platform ensures that with and without hydrogen/helium dilution, the hydrodynamics of the combustor stays fixed. Base fuel ethylene is supplemented with hydrogen or helium by the same amount to separate the dilution affects and assess the direct chemical interaction of hydrogen related to soot formation. Soot volume fraction and primary soot particle diameters were measured by auto-compensating laser induced-incandescence for all cases. Flow field data obtained using stereoscopic particle image velocimetry is utilized to ascertain the hydrodynamic effects on soot distribution due to addition of lighter species. Soot formation was found to be enhanced by the addition of hydrogen when allowance was made for the dilution effect using the helium doped flame experiments. Possible causes of this observation including the molecular diffusivities of hydrogen and helium, and chemical interaction are discussed.     

Effect of flash boiling injection on combustion and PN emissions of DISI optical engine fueled with butanol isomers/TPRF blends

Direct injection spark ignition (DISI) engines have been widely used in passenger cars due to their lower fuel consumption, better controllability, and high efficiency. However, DISI engines are suffering from wall wetting, imperfect mixture formation, excess soot emissions, and cyclic variations. Applying a new fuel atomization technique and using biofuels with their distinctive properties can potentially aid in improving DISI engines. In this research, the effects of isobutanol and 2-butanol and their blends with Toluene Primary Reference Fuel (TPRF) on spray characteristics, DISI engine combustion, and particle number (PN) emissions are investigated for conditions with and without flash boiling of the injected fuel. Spray characteristics are investigated using a constant volume chamber. Then, the combustion, flame propagation, and PN emissions are examined using an optical DISI engine. The fuel temperature is set to 298 K and 453 K for liquid injection and flash boiling injection, respectively. The tested blending ratio is 30 vol% butanol isomers and 70 vol% TPRF. The results of the spray test reveal that liquid fuel plumes are distinctly observed, and butanol blends show a slightly wider spray angle with lower penetration length compared to TPRF. However, under flash boiling injection, the sprays collapse towards the injector axis, forming a more extended single central vapor jet due to the plumes' interaction. Meanwhile, butanol blends yield a narrow spray angle with more extended penetration compared to TPRF. The flame visualization test shows that the flash boiling injection reduced yellow flames compared to liquid fuel injection, reflecting the improvements in mixture formation. Thus, improvements were noted in the heat release and PN emissions. Butanol addition reduced the PN emissions by 43% under regular liquid injection. Flash boiling injection provided an additional 25% reduction in PN emissions.     

Ability of Particulate Matter Index to describe sooting tendency of various gasoline formulations in a stratified-charge spark-ignition engine

This study investigates the ability of Particulate Matter Index (PMI) to describe the sooting behavior of various gasoline formulations in a stratified-charge (SC) spark-ignition engine. The engine was operated at 2000 rpm with an intake pressure of 130 kPa where soot formation is known to primarily occur in the bulk gases. Exhaust soot emissions were measured for nine test fuels at various exhaust gas recirculation levels. A comparison between measured soot levels and PMI shows that PMI is a relatively poor predictor of the sooting tendency of the tested fuels under lean SC combustion. Among the fuels, three fuels, namely the di-isobutylene blend, High Olefin, and E30 fuels exhibit measured soot behavior opposite of that predicted by PMI. Optical diagnostics were utilized to further investigate the in-cylinder phenomena for these three fuels. Analysis of natural luminosity and diffused back-illumination extinction imaging suggests that fuel-induced differences in the amount of soot formed are responsible for a majority of the discrepancy in measured versus predicted sooting tendency. Fuel-induced differences in soot oxidation and spray development seem to play minor roles. Because the combustion and air-fuel mixing processes for lean SC combustion are different from conventional stoichiometric operation it is hypothesized that the PMI correlation needs to be modified to account for differences in stoichiometric air-fuel ratio and level of oxygenation between fuels. Furthermore, the role of fuel volatility in PMI possibly needs to be de-emphasized for SC operation with fuel injection into compression-heated gases.     

Blending effect of methanol on the formation of polycyclic aromatic hydrocarbons in the oxidation of toluene

Methanol has been considered as a potential renewable liquid fuel and blending it with gasoline and diesel is an effective way to reduce greenhouse gas emissions from the transport sector. To understand the mixing effect of methanol on the formation of polycyclic aromatic hydrocarbons (PAHs) and oxygenated PAHs (OPAHs), the fuel-rich oxidation of toluene with and without methanol was studied using a flow reactor at atmospheric pressure, temperatures from 1050 to 1350 K, equivalence ratio of 9.0, and residence time of 1.2 s. The blending ratio of methanol was varied as 0% and 50% on a molar basis. Gas chromatograph mass spectrometer was employed to identify and quantify PAHs and OPAHs in gaseous products. A kinetic model on PAH growth up to five ring structures was used to investigate the blending effect on PAH and OPAH formation. Both experiment and modeling showed that PAH and OPAH production at lower temperatures was unexpectedly promoted in toluene/methanol oxidation compared with toluene oxidation, while their production in toluene oxidation was identical with or larger than that in toluene/methanol oxidation at elevated temperatures. In methanol oxidation, no PAHs were produced under the current experimental conditions. Kinetic analysis indicated that high methanol reactivity produced several radicals, such as OH, H, and HO₂, which promoted toluene reactivity at lower temperatures, resulting in the enlargement of PAH and OPAH formation in toluene/methanol oxidation compared to neat toluene oxidation. When the temperature was increased, the effect of methanol blending was diminished based on the kinetic analysis. These results suggest that oxygenated fuels do not necessarily reduce PAH production, but promote it under some conditions.     

Low- and high-temperature study of n-heptane combustion chemistry

n-Heptane has been used extensively in various fundamental combustion experiments as a prototypical hydrocarbon fuel. While the formation of polycyclic aromatic hydrocarbon (PAH) in n-heptane combustion has been studied preferably in premixed flames, this study aims to investigate the combustion chemistry of n-heptane in less-studied diffusion flame and highly rich high-temperature homogeneous oxidation configurations by using a counterflow burner and a flow reactor, respectively. This work addresses the formation of higher-molecular species in the mass range up to about 160 u in both configurations. Samples are analyzed by time-of-flight (TOF) molecular beam mass spectrometry (MBMS) using electron-impact (EI) and single-photon ionization (PI). Highly resolved speciation data are reported. Laminar flow reactor experiments cover a wide temperature range. Especially the measurements at low temperatures provide speciation data of large oxygenates produced in the low-temperature oxidation of n-heptane, which are scarce in the literature. Important precursor molecules for PAH and soot formation, such as C₉H₈, C₁₀H₈, C₁₁H_10, and C₁₂H₈, are formed during the high-temperature combustion process in the counterflow flame, while oxygenated growth species are observed under low-temperature conditions, even at the fuel-rich equivalence ratio of ?=4.00.Numerical modeling for both conditions is performed by using a newly developed kinetic model of n-heptane, which includes the n-heptane and PAH formation chemistry with state-of-the-art kinetic knowledge. Good agreement between model predictions and experimental data of counterflow flame and flow reactor is observed for the major species and some intermediates of n-heptane oxidation. While the concentrations of benzene and toluene measured in the counterflow burner are well-reproduced, the numerical results for flow reactor data are not satisfactory. Differences are found between the formation pathways of fulvene, from whose isomerization benzene is produced in diffusion flame and flow reactor.     

Challenges for turbulent combustion

Turbulent combustion will remain central to the next generation of combustion devices that are likely to employ blends of renewable and fossil fuels, transitioning eventually to electrofuels (also referred to as e-fuels, powerfuels, power-to-x, or synthetics). This paper starts by projecting that the decarbonization process is likely to be very slow as guided by history and by the sheer extent of the current network for fossil fuels, and the cost of its replacement. This transition to renewables will be moderated by the advent of cleaner engines that operate on increasingly cleaner fuel blends. A brief outline of recent developments in combustion modes, such as gasoline compression ignition for reciprocating engines and sequential combustion for gas turbines, is presented. The next two sections of the paper identify two essential areas of development for advancing knowledge of turbulent combustion, namely multi-mode or mixed-mode combustion and soot formation. Multi-mode combustion is common in practical devices and spans the entire range of processes from transient ignition to stable combustion and the formation of pollutants. A range of burners developed to study highly turbulent premixed flames and mixed-mode flames, is presented along with samples of data and an outline of outstanding research issues. Soot formation relevant to electrofuels, such as blends of diesel-oxymethylene ethers, hydrogen-methane or ethylene-ammonia, is also discussed. Mechanisms of soot formation, while significantly improved, remain lacking particularly for heavy fuels and their blends. Other important areas of research, such as spray atomization, turbulent dense spray flames, turbulent fires, and the effects of high pressure, are briefly mentioned. The paper concludes by highlighting the continued need for research in these areas of turbulent combustion to bring predictive capabilities to a level of comprehensive fidelity that enables them to become standard reliable tools for the design and monitoring of future combustors.     

Jet-stirred reactor and flame studies of propanal oxidation

There is a strong drive towards utilizing oxygenated biofuels in blends with existing fossil fuels. Improving the kinetic modeling of the oxidation of these bio-derived oxygenates requires further investigation of their key stable intermediates such as the aldehydes. In this study, an experimental and chemical kinetic modeling investigation of propanal oxidation was carried out. Experiments were conducted in a jet stirred reactor and in counterflow flames over a wide range of equivalence ratios, temperatures, and ambient pressures. Stable species concentration profiles were measured in the jet stirred reactor and laminar flame speeds were measured. A detailed chemical kinetic reaction model was validated using the present experimental results and existing literature data. The model was used also to provide insight into the controlling reaction pathways for propanal oxidation in both the low- and high-temperature kinetic regimes.     

An experimental investigation on spray, ignition and combustion characteristics of biodiesels

Spray, ignition and combustion characteristics of biodiesel fuels were investigated under a simulated diesel-engine condition (885 K, 4 MPa) in a constant volume combustion vessel. Two biodiesel fuels originated from palm oil and used cooking oil were used while JIS#2 used as the base fuel. Spray images were taken by a high speed video camera by using Mie-scattering method to measure liquid phase penetration and liquid length. An image intensifier combined with OH filter was used to obtain OH radical image near 313 nm. Ignition and combustion characteristics were studied by OH radical images. Biodiesel fuels give appreciably longer liquid lengths and shorter ignition delays. At low injection pressure (100 MPa), biodiesel fuels give shorter lift-off lengths than those of diesel. While at high injection pressure (200 MPa), the lift-off length of biodiesel fuel originated palm oil gives the shortest value and that of biodiesel from used cooking oil gives the longest one. Air entrainment upstream of lift-off length of three fuels was estimated and compared to soot formation distance. This study reveals that the viscosity and ignition quality of biodiesel fuel have great influences on jet flame structure and soot formation tendency.     

Soot emission reduction in oxygenated co-flow jet flames

A computational study was performed for ethylene/air non-premixed laminar co-flow jet flames using an axisymmetric CFD code to explore the effect of oxygenation on PAH and soot emissions. Oxygenated flames were established using N₂ diluted fuel stream along with O₂ enriched air stream such that the stoichiometric mixture fraction (Ζ_st) is varied but the adiabatic flame temperature is not materially changed. Simulations were carried out using a spatially and temporally accurate algorithm with detailed chemistry and transport. A detailed kinetic model involving 111 species and 784 reactions and a fairly detailed soot model were incorporated into the code. Two different approaches, one with constant flame height and other with constant inlet velocity are comprehensively examined to bring out the effects of changes in flame structure and residence time on soot emissions with respect to Z_st. With increase in Ζ_st, a drastic reduction in the formation of soot precursors (acetylene and benzene) and thus in soot emissions are observed. In the present study, oxygenated flames with Ζ_st ≥ 0.424 are considered as blue flames or completely soot free. For various oxygenated flames a C/O ratio between 0.45 and 0.6 is found to be most favorable for soot formation.     

An experimental laminar flame investigation of dual-fuel mixtures of C4 methyl esters with C2–C4 hydrocarbon base fuels

Blending petroleum-based fuels with biofuel components is deemed attractive to reduce soot and CO₂ emissions, but fundamental studies of the combustion behavior of such fuel blends suited for model development and validation remain rather scarce. To contribute to the understanding of the combustion chemistry effects of such blending strategies, we have investigated laminar premixed low-pressure flames of three hydrocarbon base fuels, namely 1-butene (1-C₄H₈), isobutene (i-C₄H₈), and ethene (C₂H₄), blended each with two different ester fuels, namely methyl crotonate (C₅H₈O₂, MC) and methyl butanoate (C₅H₁₀O₂, MB). A series of 13 flames with different argon dilution was investigated to study effects of the specific fuel structure on the combustion chemistry. Full speciation analyses were performed for fuel-rich (? = 1.6) conditions by electron ionization molecular-beam mass spectrometry (EI-MBMS). More than 35 species in the range of C₀–C₇ were identified and quantified in these flames, resulting in ～450 mol fraction profiles. The experimental data were compared to simulations by the kinetic model reported by Yang et al. [Proc. Combust. Inst. 2011, Phys. Chem. Chem. Phys. 2013] that was chosen because it includes basic mechanisms of all studied fuels. Overall, the agreement of experiment and this model seems satisfactory but calls for further improvements regarding ester as well as hydrocarbon sub-mechanisms. It was noted that the unsaturation degree in the methyl esters affects the formation of hydrocarbons, that depend mainly on the structure of the respective base fuel, and of oxygenated intermediates. The methyl esters have different decomposition pathways leading to some specific oxygenated species. Both methyl esters promote the formation of formaldehyde and methanol, while acetic acid is significantly increased by the presence of MB. The effect of the ester addition is also influenced by the species pool of the respective hydrocarbon base fuel.     

Detailed investigation of soot formation from jet fuel in a diffusion flame with comprehensive and hybrid chemical mechanisms

A numerical and experimental study is performed to investigate soot formation from jet fuel in a laminar coflow diffusion flame. The combustion chemistry of the fuel is simulated using (1) the MURI jet fuel surrogate (Dooley et al. 2012) with a modestly reduced Ranzi mechanism (Ranzi et al. 2012), and (2) the recently proposed HyChem model (Xu et al. 2018) combined with the KAUST PAH mechanism 2 (Wang et al. 2013). The two reaction mechanisms are coupled with a sectional soot model to simulate a coflow diffusion flame of methane doped with the MURI jet fuel surrogate. The combined laser extinction and two-angle elastic light scattering method is used to perform non-intrusive in situ measurements of soot volume fraction, primary particle diameter and number density. The good agreement including soot particle size and number density between the experimental data and the simulation results computed with the reduced Ranzi mechanism demonstrate the robustness of the soot model to changes in fuel composition, as the model parameters are unchanged with a previous numerical study of soot formation of n-propylbenzene/n-dodecane mixtures (Zhang and Thomson, 2018). The computation with the combined HyChem/KAUST mechanism predicts similar results as the computation with the detailed chemistry of the reduced Ranzi mechanism for fuel breakdown, thus the basic premise of the HyChem model that the fuel decomposition process can be greatly simplified with the lumped reaction steps is supported. The results also show that by adding a PAH growth scheme to the HyChem model, the approach can be used to predict soot formation from jet fuel combustion in a laminar coflow diffusion flame. Finally, the dependency of the soot prediction on PAH chemistry is discussed and it is suggested that more experimental data is needed to validate the PAH mechanism and improve the predictive accuracy of the model.     

Modeling of flame lift-off length in diesel low-temperature combustion with multi-dimensional CFD based on the flame surface density and extinction concept

Low-Temperature Combustion (LTC) is becoming a promising technology for simultaneously reducing soot and NOx emissions from diesel engines. LTC regimes are evaluated by the flame lift-off length – the distance from the injector orifice to the location of hydroxyl luminescence closest to the injector in the flame jet. Various works have been dedicated to successful simulations of lifted flames of a diesel jet by use of various combustion modeling approaches. In this work, flame surface density and flamelet concepts were used to model the diesel lift-off length under LTC conditions. Numerical studies have been performed with the ECFM3Z model, n-Heptane and diesel fuels to determine the flame lift-off length and its correlation with soot formation under quiescent conditions. The numerical results showed good agreement with experimental data, which were obtained from an optically accessible constant volume chamber and presented at the Engine Combustion Network (ECN) of Sandia National Laboratories. It was shown that at a certain distance downstream from the injector orifice, stoichiometric scalar dissipation rate matched the extinction scalar dissipation rate. This computed extinction scalar dissipation rate correlated well with the flame lift-off length. For the range of conditions investigated, adequate quantitative agreement was obtained with the experimental measurements of lift-off length under various ambient gas O₂ concentrations, ambient gas temperatures, ambient gas densities and fuel injection pressures. The results showed that the computed lift-off length values for most of the conditions lay in a reasonable range within the quasi-steady lift-off length values obtained from experiments. However, at ambient temperatures lower than 1000 K, the lift-off length values were under-predicted by the numerical analysis. This may be due to the use of the droplet evaporation model as it is believed that evaporation has a strong effect on the lift-off length.     

Modeling soot formation in flames and reactors: Recent progress and current challenges

The study of soot has long been motivated by its adverse impacts on health and the environment. However, this combustion knowledge is also relevant to the production of carbon black and hydrogen via methane pyrolysis which are important commodities. Over the last decade, steady progress has been made in the development of detailed continuum models of soot formation in flames and reactors. Developing more comprehensive models has often been motivated by the need for predicting soot formation over a wider range of conditions (e.g., temperature, pressure, fuels). Measurements with novel experimental techniques have given us new insights into the chemistry, particle dynamics and optical properties of soot particles and even molecules and radicals forming them. Also, multi-scale modeling has enabled us to translate the detailed mechanisms of soot processes based on first principles into computationally efficient but accurate continuum models of soot formation in flames and reactors. However, important questions remain including (1) what is the mechanism of soot inception and surface growth, (2) which gas-phase species are involved in soot inception and surface growth (3) how surface growth and oxidation are affected by soot surface properties. Proposed models need to be evaluated against experimental data over a wide range of conditions to determine their predictive strength. These questions are critical for the accurate prediction of soot formation in flames and its emissions from engines. However, this knowledge can also be used to develop predictive process design and optimization tools for carbon black and other nanocarbon formation in reactors.     

Water vapour effects on temperature and soot loading in ethylene flames in hot and vitiated coflows

Flames in hot, low oxygen environments exist in a variety of practical applications. These conditions result in significant mixing between fuel and combustion products, such as water vapour, or diluents included for emissions control. The chemical and physical effects of water vapour as a diluent are investigated in a series of ethylene flames in a jet in hot coflow burner to determine the effects on temperature and soot fields. The combined analyses of photographs, non-linear excitation regime two-line atomic fluorescence (NTLAF) of indium, planar laser-induced incandescence (LII) and one-dimensional opposed-flow flame simulations demonstrate the dominance of the chemistry, driven by the hot and vitiated oxidant, in soot reduction. Although photographs appear to suggest that both highly vitiated coflows, and highly diluted jet flames have global effect on the flames, detailed measurements reveal significantly different trends in their soot and temperature fields. The chemical contribution of water vapour as a reactant, as a third-body in ethylene decomposition and a source of H and OH in the rich mixture is further described, and trends subsequently identified, in the context of formation of polycyclic aromatic hydrocarbons and soot reduction.     

Application of an aerosol shock tube to the measurement of diesel ignition delay times

Shock tube ignition delay times were measured for DF-2 diesel/21% O₂/argon mixtures at pressures from 2.3 to 8.0 atm, equivalence ratios from 0.3 to 1.35, and temperatures from 900 to 1300 K using a new experimental flow facility, an aerosol shock tube. The aerosol shock tube combines conventional shock tube methodology with aerosol loading of fuel-oxidizer mixtures. Significant efforts have been made to ensure that the aerosol mixtures were spatially uniform, that the incident shock wave was well-behaved, and that the post-shock conditions and mixture fractions were accurately determined. The nebulizer-generated, narrow, micron-sized aerosol size distribution permitted rapid evaporation of the fuel mixture and enabled separation of the diesel fuel evaporation and diffusion processes that occurred behind the incident shock wave from the chemical ignition processes that occurred behind the higher temperature and pressure reflected shock wave. This rapid evaporation technique enables the study of a wide range of low-vapor-pressure practical fuels and fuel surrogates without the complication of fuel cracking that can occur with heated experimental facilities. These diesel ignition delay measurements extend the temperature and pressure range of earlier flow reactor studies, provide evidence for NTC behavior in diesel fuel ignition delay times at lower temperatures, and provide an accurate data base for the development and comparison of kinetic mechanisms for diesel fuel and surrogate mixtures. Representative comparisons with several single-component diesel surrogate models are also given.     

Experimental and semi-detailed kinetic modeling study of decalin oxidation and pyrolysis over a wide range of conditions

Decalin is the simplest polycyclic alkane (polynaphtenic hydrocarbon) found in liquid fuels (jet fuels, Diesel). In order to better understand the combustion characteristics of decalin, this study provides new experimental data for its oxidation in a jet-stirred reactor. For the first time, stable species concentration profiles were measured in a jet-stirred reactor at a constant mean residence time of 0.1 s and 0.5 s at respectively 1 and 10 atm, over a range of equivalence ratios (? = 0.5–1.5) and temperatures (750–1350 K). The oxidation of decalin under these experimental conditions was modeled using a semi-detailed chemical kinetic reaction mechanism (11,000 reactions involving 360 species) derived from a previously proposed scheme for the ignition of the same fuel in a shock-tube. The proposed mechanism that includes both low- and high-temperature chemistry shows reasonably good agreement with the present experimental data set. It can also represent well decalin pyrolysis and oxidation data available in the literature. Reaction path analyses and sensitivity analyses were conducted to interpret the results.     

Surrogate formulation for diesel and jet fuels using the minimalist functional group (MFG) approach

Surrogate fuels aim to reproduce real fuel combustion characteristics in order to enable predictive simulations and fuel/engine design. In this work, surrogate mixtures were formulated for three diesel fuels (Coryton Euro and Coryton US-2D certification grade and Saudi pump grade) and two jet fuels (POSF 4658 and POSF 4734) using the minimalist functional group (MFG) approach, a method recently developed and tested for gasoline fuels. The diesel and jet fuel surrogates were formulated by matching five important functional groups, while minimizing the surrogate components to two species. Another molecular parameter, called as branching index (BI), which denotes the degree of branching was also used as a matching criterion. The present works aims to test the ability of the MFG surrogate methodology for high molecular weight fuels (e.g., jet and diesel). ¹H Nuclear Magnetic Resonance (NMR) spectroscopy was used to analyze the composition of the groups in diesel fuels, and those in jet fuels were evaluated using the molecular data obtained from published literature. The MFG surrogates were experimentally evaluated in an ignition quality tester (IQT), wherein ignition delay times (IDT) and derived cetane number (DCN) were measured. Physical properties, namely, average molecular weight (AMW) and density, and thermochemical properties, namely, heat of combustion and H/C ratio were also compared. The results show that the MFG surrogates were able to reproduce the combustion properties of the above fuels, and we demonstrate that fewer species in surrogates can be as effective as more complex surrogates. We conclude that the MFG approach can radically simplify the surrogate formulation process, significantly reduce the cost and time associated with the development of chemical kinetic models, and facilitate surrogate testing.